Browsing by Author "Zhang, Xusheng"

Sterols and their conjugates are minor natural constituents in oils and fats. Plant sterols are also enriched in functional foods because of their known ability to lower serum cholesterol levels. Like other unsaturated lipids, sterols are prone to oxidize. In the literature review, thermo-oxidation reactions of free and esterified sterols were introduced and methods for the analysis of the formed products were summarized. The aim of the experimental research was to study the oxidation reactions of steryl esters. The oxidation reactions were studied at two temperatures, at 100 ºC and 180 ºC, by following the changes in the profiles of oxidation products varying in polarity and molecular size. The effects of esterification, unsaturation level of the acyl moiety in the sterol structure and temperature were studied. Solid-phase extraction (SiOH-SPE) and size-exclusion chromatography (SEC-RI) were applied for the separation and quantification of monomeric and oligomeric oxidation products. Silver coordination ion spray mass spectrometry (Ag+-CIS-MS) was applied to indicate mass ranges of the formed oxidation products. In the oxidation, sterol molecules first reacted with oxygen and then the formed monomeric oxidation products began to link with each other forming oligomers. Oligomers contributed to 17-90% and 23-78% of the oxidation products of steryl esters after 3 days and 2 hours of heating at 100 ºC and 180 ºC, respectively. Polar oligomers were the most abundant fraction at both temperatures. Non-polar oligomers were not formed in the oxidation of steryl esters. Increased unsaturation of the acyl moiety increased polymerization. Increased temperature changed the oxidation reactions and mid-polar oligomers became more dominant. Thermal reactions led to the loss of hydroxyl groups and hence the polarity of oligomers decreased. The molecular weight ranges of the most abundant oligomers and their fragments were m/z 900-1100 and 1300-1500 at 100 ºC. In conclusion, the oxidation of steryl esters begins by reactions with molecular oxygen after which the formed oxidation products begin to polymerize. This occurs both at auto-oxidation and thermo-oxidation conditions; i.e., at cooking and frying temperatures.